In recent years, it is increasingly necessary to reduce the size of circuit patterns for high integration of a large-scale integrated circuit device (hereinafter, referred to as “LSI”) that can be achieved with semiconductors. As a result, reduction of the width of a line for wiring patterns constituting a circuit or reduction of the size of contact hole patterns (hereinafter, referred to as “contact patterns”) that connect between layered wirings formed with an insulating layer therebetween have become very important.
Hereinafter, reduction of the size of wiring patterns and contact patterns with a conventional light-exposure system will be described by using a positive resist process as an example. In a positive resist process, a line pattern refers to part of a resist film (resist pattern) which is not exposed with exposure light, i.e., part thereof which is left after development. A space pattern refers to part of a resist film which is exposed with exposure light, i.e., an opening portion which is formed by removing part of a resist film by development (resist-removed pattern). A contact pattern refers to a hole-like opening which is formed by removing part of a resist film by development and can be regarded as a small space pattern of the space patterns. When using a negative resist process instead of a positive resist process, the definition of the line pattern and the definition of the space pattern are replaced by each other.
<First Conventional Example>
As a conventional method for forming a thin line pattern, a method for forming a line pattern with a very small width by enhancing, using a phase shifter, a contrast of a light-intensity distribution generated by a mask pattern has been proposed (e.g., H. Y Liu et al., Proc. SPIE, Vol. 3334, P.2 (1998)).
Hereinafter, a conventional method for forming a line pattern using a phase shifter will be described with reference to the accompanying drawings.
FIG. 28(a) illustrates an exemplary layout for a desired pattern (resist pattern) to be formed. As shown in FIG. 28(a), a pattern 800 includes a partial pattern 800a having a predetermined dimension or less.
FIGS. 28(b) and 28(c) are plan views of two conventional photomasks used for forming the pattern shown in FIG. 28(a), respectively. As shown in FIG. 28(b), in a first photo mask 810, a complete-light-shielding film 812 (having a transmittance of about 0% with respect to exposure light) is formed on a transparent substrate 811. Moreover, a first opening 813 which is to be a light-transmitting portion and a second opening portion 814 which is to be a phase shifter are provided on the complete-light-shielding film 812 with a light-shielding pattern 812a for forming the partial pattern 800a interposed between the first and second openings 813 and 814. The second opening 814 which is to be a phase shifter transmits exposure light to generate a phase difference of 180 degrees with respect to the first opening 813 which is to be a light-transmitting portion. As shown in FIG. 28(c), in a second photomask 820, a light-shielding pattern 822 for forming the desired pattern 800 (see FIG. 28(a)) by combination with a light-shielding pattern 812a of the first photomask 810 is formed on a transparent substrate 821.
A method for forming a pattern using the two photomasks shown in FIGS. 28(a) and 28(b) is as follows. First, using the first photomask 810, a substrate to which a resist film made of a positive resist is applied is exposed with exposure light. Thereafter, alignment of the second photomask 820 is performed so that the pattern 800 of FIG. 28(a) is formed, and then the substrate is exposed with light using the second photomask 820. Thereafter, the resist film is developed, thereby forming a resist pattern shown in FIG. 28(a). In this case, an excess pattern (i.e., patterns other than the pattern 800) which is to be left when exposure is performed using only the first photomask 810 can be removed by the exposure using the second photomask 820. Accordingly, the partial pattern 800a which has a very small width and can not be formed only by exposure process using only the second photomask 820 can be formed.
In this method, if a light-transmitting portion and a phase shifter are disposed with a pattern (i.e., a light-shielding pattern) made of a complete-light-shielding film having a predetermined dimension or less interposed therebetween, lights transmitted through the light-transmitting portion (opening) and the phase shifter, respectively, and diffracted at the back side of the light-shielding pattern are cancelled with each other, improving light-shielding properties of the light-shielding pattern. Thus, a line pattern with a predetermined dimension or less can be formed.
<Second Conventional Example>
As a method for forming a conventional small contact pattern, a method in which a half-tone phase-shifting mask is used has been proposed. In the half-tone phase-shifting mask, a light-transmitting portion (i.e., an opening in a phase shifter) corresponding to a contact pattern is provided. Moreover, as the light-shielding portion, provided is a phase shifter which has a low transmittance (of about 3%–6%) with respect to exposure light and transmits exposure light with a phase inversion of 180 degree with respect to exposure light transmitted through the opening.
Hereinafter, a principle of a pattern forming method using the half-tone phase-shifting mask will be described with reference to FIGS. 29(a) through 29(g).
FIG. 29(a) is a plan view of a photomask in which an opening corresponding to a contact pattern is provided in a chromium film which is to be a complete-light-shielding portion provided on the surface of the mask. FIG. 29(b) shows the amplitude intensity of light transmitted through the photomask of FIG. 29(a) and transferred onto a position corresponding to the line AA′ on a material to be exposed. FIG. 29(c) is a plan view of a photomask in which the chromium film corresponding to a contact pattern is provided as a complete-light-shielding portion in a phase shifter provided on the surface of the mask. FIG. 29(d) shows the amplitude intensity of light transmitted through the photomask of FIG. 29(c) and transferred onto a position corresponding to the line AA′ on a material to be exposed. FIG. 29(e) is a plan view of a photomask (i.e., half-tone phase-shifting mask) in which an opening corresponding to a contact pattern is provided in a phase shifter which is to be a light shielding portion provided on the surface of the mask. FIGS. 29(f) and 29(g) show the amplitude intensity and light intensity of light transmitted through the photomask of FIG. 29(e) and transferred onto a position corresponding to the line AA′ on a material to be exposed.
As shown in FIGS. 29(b), 29(d) and 29(f), the amplitude intensity of light transmitted through the half-tone phase-shifting mask of FIG. 29(e) is the sum of the amplitude intensities of lights transmitted through the respective photomasks of FIGS. 29(a) and 29(c). More specifically, in the half-tone phase-shifting mask of FIG. 29(e), the phase shifter which is to be a light-shielding portion is configured so as to not only transmit part of exposure light but also provide a phase difference of 180 degrees, with respect to light transmitted through the opening, to light transmitted through the phase shifter. Therefore, as shown in FIGS. 29(b) and 29(d), the light transmitted through the phase shifter has an amplitude intensity distribution with a phase opposite to that of the light transmitted through the opening. Thus, when the amplitude intensity distribution shown in FIG. 29(b) and the amplitude intensity distribution shown in FIG. 29(d) are synthesized, a phase boundary in which the amplitude intensity is turned to 0 by a phase change is generated, as shown in FIG. 29(f). As a result, as shown in FIG. 29(g), in the end of the opening that is to be the phase boundary (hereinafter, referred to as a “phase end”), the light intensity, which is represented by a square of the amplitude intensity, becomes 0, and a significantly dark portion is formed. Accordingly, in an image of the light transmitted through the half-tone phase-shifting mask shown in FIG. 29(e), a strong contrast is realized in the periphery of the opening. Therefore, a small contact pattern can be formed.
A light source used for exposure herein will be described. FIGS. 30(a) through 30(c) are illustrations showing shapes of light sources which have been conventionally used for exposure. In contrast with a regular exposure light source shown in FIG. 30(a), off-axis exposure light source is a light source shown in FIGS. 30(b) and 30(c) in which a light element entering vertically to part of a photomask corresponding to the light source center is removed. Typical off-axis exposure light sources includes an annular exposure light source shown in FIG. 30(b) and a quadrupole exposure light source shown in FIG. 30(c). Although it slightly depends on a desired pattern, in general, quadrupole exposure light sources are more advantageous in enhancement of the contrast and enlargement of the DOF (depth of focus) than annular exposure light sources.
However, the pattern forming method of the first conventional example has had the following problems.
(1) When a light-shielding, pattern is interposed between a light-transmitting portion and a phase shifter to improve the contrast of an image corresponding to the light-shielding pattern, the light-transmitting portion and the phase shifter have to adjoin each other with a distance of a predetermined dimension or less therebetween. On the other hand, when the light-transmitting portion and the phase shifter are disposed with no light-shielding pattern interposed therebetween on a photomask, an image corresponding to the boundary between the light-transmitting portion and the phase shifter is formed. Accordingly, if only the first photomask shown in FIG. 28(b) is used, a pattern having an arbitrary shape can not be formed. Therefore, in order to form a pattern having a complicated shape such as a pattern layout of a regular LSI, it is necessary to perform exposure using not only a first photomask shown in FIG. 28(b) but also a second photomask shown in FIG. 28(c). As a result, costs for masks are increased and also the number of process steps in a lithograph process is increased to cause reduction in throughput or increase in production costs.
(2) When a desired (resist) pattern has a complicated shape (e.g., a T-shaped having a predetermined dimension or less) is intended to be formed, a whole light-shielding pattern can not be provided only between a light-transmitting portion and a phase shifter having phases opposite to each other. Thus, light-shielding properties of a T-shaped light-shielding pattern, for example, can not be improved. Therefore, a pattern layout with which effects of the phase shifter can be utilized is limited.
Moreover, the pattern forming method of the second conventional example has had the following problems.
(3) Depending on a half-tone phase-shifting mask, it is difficult to simultaneously form an isolated contact pattern in which contacts are arranged so as to be isolated from each other and a densely arranged contact pattern in which contacts are densely arranged by exposure using the same light source and with sufficient finishing quality. In the same manner, it is difficult to simultaneously form an isolated line pattern in which lines are arranged so as to be isolated from each other and a densely arranged line pattern in which lines are densely arranged by exposure using the same light source and with sufficient finishing quality. More specifically, assume that the isolated contact pattern is formed. If vertical incident exposure is performed with the same small light source having a low coherence degree of about 0.5 or less (see FIG. 30(a)) and being used for illumination by only vertical incident components that enter vertically to a mask, an improved contrast and increased depth of focus can be achieved. However, if vertical incident exposure is used to form the densely arranged contact pattern, the contrast and the depth of focus are significantly deteriorated. On the other hand, assume that the densely arranged contact pattern is formed. If off-axis illumination (oblique incident exposure) is performed using a light source being used for illumination by only off-axis components that enter obliquely to a mask, e.g., a light source for annular illumination in which vertical incident components (illumination components from the light source center) are removed (see FIG. 30(b)), an improved contrast and increased depth of focus can be achieved. However, if off-axis exposure is used to form the isolated line pattern, the contrast and the depth of focus are significantly deteriorated.
(4) Depending on the half-tone phase-shifting mask, it is difficult to simultaneously form an isolated space pattern and an isolated line pattern with sufficient finishing quality. More specifically, when the isolated space pattern is formed, an improved contrast and increased depth of focus can be achieved by performing vertical incident exposure. However, if vertical incident exposure is used to form the isolated line pattern, the contrast and the depth of focus are significantly deteriorated. On the other hand, when the isolated line pattern is formed, an improved contrast and increased depth of focus can be achieved by performing off-axis exposure However, if off-axis exposure is used to form the isolated space pattern, the contrast and the depth of focus are significantly deteriorated. As has been described, when the half-tone phase-shifting mask is used, optimal illumination conditions for isolated space patterns (including an isolated contact pattern) and optimal illumination conditions for densely arranged space patterns (including a densely arranged contact pattern) or isolated line patterns have a contradictory relationship. Therefore, it is difficult to form an isolated space pattern simultaneously with an isolated line pattern or a densely arranged space pattern under the same illumination conditions and with optimal finishing quality.